(a) Technical Field of the Invention
The present invention generally relates to electro-mechanical switches, more particularly to micro-electromechanical switches (MEMS), and most particularly to high power RF MEMS.
(b) Description of Related Art
In communications applications, switches are often designed with semiconductor elements such as transistors or pin diodes. At microwave frequencies, however, these devices suffer from several shortcomings. Pin diodes and transistors typically have an insertion loss greater than 1 dB, which is the loss across the switch when the switch is closed. Transistors operating at microwave frequencies tend to have an isolation value less than 20 dB. This allows a signal to ‘bleed’ across the switch even when the switch is open. Pin diodes and transistors have a limited frequency response and typically only respond to frequencies below 20 GHz. In addition, the insertion losses and high isolation value for these switches vary depending on the frequency of the signal passing through the switches. These characteristics make semiconductor transistors and pin diodes a poor choice for switches in microwave applications.
U.S. Pat. No. 5,121,089, to Larson, disclosed a new class of microwave switch—the micro-electro-mechanical (MEM) switch. The MEM switch has a very low insertion loss (less than 0.2 dB at 45 GHz) and a high isolation when open (greater than 30 dB). In addition, the switch has a large frequency response and a large bandwidth compared to semiconductor transistors and pin diodes. These characteristics give the MEM switch the potential to replace traditional narrow-bandwidth PIN diodes and transistor switches in microwave circuits.
The Larson MEM switch utilizes an armature design. One end of a metal armature is affixed to an output line, and the other end of the armature rests above an input line. The armature is electrically isolated from the input line when the switch is in an open position. When a voltage is applied to an electrode below the armature, the armature is pulled downward and contacts the input line. This creates a conducting path between the input line and the output line through the metal armature.
Micro-electromechanical switches of the general type described above are, however, prone to premature failure. The cause of the premature failure is linked to the damage resulting from the impact of the armature contact with the substrate contact. This damage is exacerbated by the fact that conventional MEM switches have armature contacts that impinge on the substrate contact surface at an angle. The angled impact results in all the impact energy being transferred to a relatively small area, thereby ultimately causing premature failure due to both increased impact per unit area and heat caused by resistive heating due to increased current density through the small area of actual contact.